In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The detectors are generally rectangular. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. Typically, the configuration of a slice may be varied. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts that attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
The x-ray source typically includes an evacuated x-ray envelope containing an anode and a cathode. X-rays are produced when electrons from the cathode are accelerated against a focal spot on the anode by applying a high voltage across the anode and cathode. The x-rays diverge from the focal spot in a generally conical pattern.
In known CT systems, the x-ray beam from the x-ray source is projected through a pre-patient collimating device, or collimator, that defines the x-ray beam profile in the patient axis, or z-axis. The collimator includes x-ray absorbing material with an aperture therein for restricting the x-ray beam. The process of restricting the x-ray beam to the desired fan beam profile is termed "collimation".
With respect to restricting the x-ray beam, known collimators typically include two opposing metallic blades that may be opened and closed to change the aperture width. The fan beam "thickness", as measured along the z-axis, can be selected by adjusting the blade orientation. The blades also may be moved in a same direction to displace the centerline of the aperture. Changing the aperture centerline changes the fan beam angle with respect to the z-axis. Known apertures are typically linear, or rectangular.
The collimated beam attenuates through a patient and the attenuated beam at least partially falls on a detector array. Known detector arrays typically include detector cells arranged in an arc configuration having a constant radius from the source. Since the collimator aperture is rectangular, an effective source to collimation distance (s) changes as the fan angle (.alpha.) at which the fan beam impinges upon the detector cells of the detector array changes. Therefore, a convex shaped collimated x-ray beam is projected on the detector. However, when a post-patient collimator is used, each detector cell typically senses only a rectangular portion of the x-ray beam umbra. A portion of the convex shaped attenuated x-ray beam is not used. A patient, therefore, is subject to unnecessary x-ray dose since a portion of the attenuated beam is unused. To reduce unnecessary x-ray dose, the collimator aperture can be narrowed. Narrowing the collimator aperture, however, also reduces the data collected by the detector.
In multislice CT systems, it is desirable to have only the umbra of the x-ray beam fall on the detector cells. Although the x-ray beam can initially be collimated so that the penumbra does not fall on the detector cells, thermal expansion of the anode support structure as the x-ray source heats up affects the alignment of the fan beam with the imaging plane. Gravitational and centrifugal forces are also known to cause focal spot movement, which also results in fan beam movement. As the fan beam moves, it is possible that at least part of the penumbra will fall on the detector cells. Movement of the fan beam changes the strength of signals from the detector array cells. Such fan beam movement may cause differential gain errors and result in severe ring, band and center artifacts unless sophisticated signal correction is employed. However, even when using perfect closed loop beam stabilization to minimize the beam movement on the detector, i.e., moving the detector or collimator, the convex beam shape results in subjecting the patient to some amount of unnecessary x-ray dose.
It would be desirable to restrict the fan beam to reduce the unnecessary x-ray dose while maintaining the amount and quality of data currently collected by detector cells. It also would be desirable to restrict the beam to more closely approximate the shape of each detector element in multislice CT systems which depend upon distinguishing between the umbra and penumbra of the fan beam.